mmm, superb. To write such poetry and have it ring just right, is no mean feat; it's internalized and made new. The initial lines are familiar, but I don't recall (in traditional works) the explicit explanation spelled out just so as at poem's end (though likely, as Sunil suggests, this is an olden mystical archetype; it's a paradox as basic in its context as, say, what became the Christian doctrine of divine sacrifice in another context).

Rarely is such a poem seen in English without an issue of basic stiltedness; -- a point this moves smoothly & well beyond.